1. Field of the Invention
The present invention generally relates to an electrochemical battery cell. More particularly, the present invention relates to designs of power terminals of an electrochemical battery cell.
2. Background
An electrochemical battery cell can be, for example, a prismatic cell or a cylindrical cell. A prismatic cell (e.g., a prismatic lithium ion cell) includes cathode and anode sheets or plates that are stacked together; while in a cylindrical cell the electrode sheets are rolled into a cylindrical structure. The electrode sheets are separated by non-conductive layer(s) and sealed hermetically within a cell enclosure. Typically, a conventional prismatic battery cell has two power terminals or extension tabs (a positive terminal and a negative terminal) disposed at one end or two opposite ends of the cell. Extension tabs can extend from current collecting tabs attached to the electrodes. The positive and negative extension tabs are typically made from different materials. For example, the extension tabs are often made of aluminum (positive) and copper (negative) or nickel (negative).
Conventional prismatic cells have extension tabs that are symmetric in size (i.e., the dimensions of the positive and negative extension tabs are identical). FIGS. 1A, 1B, and 1C illustrate prior art prismatic cells 102, 104, and 106, respectively. As shown, the dimensions of the extension tabs (terminals 112a, 112b for cell 102, terminals 114a, 114b for cell 104, and terminals 116a, 116b for cell 106) are roughly identical for each cell.
When electrical currents on the positive extension tab and the negative extension tab of a cell are high enough, joule heating in the two extension tabs becomes significant relative to heat transfer by thermal conduction out of the terminals. Because the two extension tabs are constructed from different materials having different electrical resistivity and thermal conductivity, heat built-up on the two extension tabs caused by the electrical currents is different. Therefore, the temperatures of symmetric extension tabs made from materials with inherently different thermal and electrical properties will not be identical, and one extension tab or terminal will have a higher temperature during the lifetime of the cell. This difference in temperature depends on the actual cycling current and the thermal environment to which the cell is exposed, but can be large and can be a limiting factor in the performance of the cell with respect to cell life and/or behavior. This problem is especially significant when the cell is subjected to abuse conditions such as a low resistance external short circuit. Hence, a cell design that reduces the differences in cell temperatures (and/or the maximum cell temperature) is desirable.